1. Field of the Invention
The present invention relates to a photographic film cassette, more particularly, the present invention relates to an improvement of a photographic film cassette in which rotation of a spool causes a leader of a photographic film to advance to the outside of a cassette shell.
2. Description of the Related Art
A known photographic film cassette includes a photographic filmstrip (hereinafter referred to as film) positioned so that a leader of the photographic film does not protrude form the cassette shell prior to loading the cassette in a camera. Such a cassette is easily loaded into a camera. A simple film-advancing mechanism of the camera is typically used with the type of cassette, and includes a construction which rotates a spool to unwind the photographic film, thereby causing the leader to move through a passageway formed in the cassette and to exit therefrom.
In the above-mentioned type of cassette, the roll of photographic film wound on the spool is prevented from loosening, when the spool is rotated, in order to transmit the rotation of the spool to the leader. In a cassette as disclosed in U.S. Pat. No. 4,846,418 and 4,887,776, the spool has a pair of spool pieces axially slidable relative to each other. A first spool pieces is provided with a first spool flange and a cam follower, whereas the second spool piece is provided with a second spool flange and a cam groove formed to receive the cam follower. When the spool pieces are rotated relative to one another, the cam follower is guided along the cam groove so as to shorten the axial interval between the two flanges by sliding the spool pieces axially relative to one another. When sliding the spool pieces, at least the outermost turn of the roll is clamped between the flanges so that the roll is integrally rotatable with the spool and is thus prevented from loosening.
The cam mechanism of the cassette described above, however, is low in strength. When the two spool pieces are assembled together to engage the cam follower with the cam groove, an auxiliary structure is necessary for convenience in inserting the cam follower into the cam groove by passing the cam follower through a sleeve of a core end of the second spool piece. The cassette disclosed in U.S. Pat. No. 4,846,418 has a cam follower with slits in two lateral sides thereof so as to allow the cam follower to flex so as to allow the cam follower to pass through the inside of the second spool piece. Although the sleeve-formed core end associated with the cam groove is strong enough, the cam follower is low in strength. The cassette disclosed in U.S. Pat. No. 4,887,776, on the other hand, has a cam groove with a slit communicating to the core end of the second spool piece. The cam follower is guided through this slit upon assembly. Although the cam follower of this device is strong, the core end associated with the cam groove is low in strength, as the cam groove is open to the core end.
Also, the cam follower may be destroyed when an excessive force is abruptly applied thereto, as the cam follower is formed to project in a long and thin shape. Once the cam follower is broken, the photographic film can be neither advanced nor wound up in the cassette shell, as a trailer of the photographic film is anchored on the second spool piece which is separate from the first spool piece. The first spool piece is rotated by the camera.
When the spool pieces have been assembled together, they may become disengaged unless both spool pieces are supported in the axial direction. The shortenable spool that is susceptible to being taken apart thus requires a cassette having an exceedingly complicated device for transporting and supporting the spool during manufacture.